1. Field of the Invention
The present invention relates to a wire bonding method in which first bonding points and second bonding points are connected by wires in a semiconductor device manufacturing process.
2. Prior Art
As shown in FIG. 4, in semiconductor devices, a wire is bonded so that first bonding points on pads 3 (31, 32 . . . 3N) of a semiconductor chip 2 mounted on a lead frame 1 and second bonding points on leads 4 (41, 42 . . . 4N) of a lead frame 1 are connected by wires 5.
This wire bonding is generally performed by the steps shown in FIG. 5.
As shown in step (a), a capillary 6 is lowered so that a ball formed at the tip end of a wire 5 is bonded to a first bonding point A; afterward, the capillary 6 is raised slightly to point B, delivering the wire 5. Next, in step (b), the capillary 6 is moved horizontally by a slight distance to point C in the opposite direction from the second bonding point E. Generally, moving a capillary in a direction opposite from a second bonding point E during a process of forming a wire loop between the first and second bonding points is called a reverse operation.
Next, in step(c), the capillary 6 is raised to point D (for a distance that corresponds to the amount of wire required for wire loop formation), delivering the wire 5. Afterward, the capillary 6 is moved to the second bonding point E in a circular-arc motion with a radius centered on the first bonding point A or a point near the first bonding point A; and then, the wire 5 is bonded to the second bonding point E.
In most cases, however, as shown in FIG. 4, the pads 3 (31, 32 . . . 3N), the first bonding points, are disposed at equal intervals inside the peripheral edges on the four sides of a semiconductor chip 2. Accordingly, if the number of connections of the wire 5 is large relative to the area of the semiconductor chip 2, the pad pitch (i. e., the distance from the center of a given pad 3 to the center of an adjacent or next pad 3) is inevitably small. In such a case, the area of the pads 3 also needs to be inevitably small.
With the miniaturization of semiconductor devices in recent years, bonding to pads 3 spaced at narrower intervals has become more and more necessary.
In cases where the spacing of pads 3 (pad pitch P1) is relatively wide, a thick capillary 6 as shown in FIG. 6 can be used. However, when the spacing of the pads 3 (pad pitch P2) is narrow, the tip of the capillary 6 may contact adjacent press-bonded balls during bonding, or the capillary 6 may contact and cause a deformation of wire loops that have already been formed. Accordingly, it is necessary to use a thin capillary 6 as shown in FIG. 7. Nonetheless, when the thickness of the capillary 6 is reduced, the mechanical strength of the capillary 6 is correspondingly weakened, so that the useful life of the capillary 6 is shortened.
Furthermore, as shown in FIGS. 6 and 7, the ball 7 formed at the tip of the wire 5 is crushed and deformed by the capillary 6 during the bonding process, and a press-bonded ball 7a is formed. The diameter of such a press-bonded ball 7a is ordinarily two to three times the diameter of the wire 5; accordingly, a spacing that keeps this press-bonded ball 7a from not contacting adjacent pads 3 or press-bonded balls 7a is required.
Accordingly, the pad pitch needs to be the largest of the following three spacings: a spacing that avoids the tip of the capillary 6 from contacting adjacent press-bonded balls 7a during bonding, a spacing that avoids the capillary 6 from contacting wire loops that have already been formed, and a spacing that avoids the press-bonded ball 7a at each bonding point from contacting adjacent pads 3 or press bonded balls 7a.
Conventionally, the bonding order, i. e., the order in which each successive pair of first and second bonding points is connected by a wire after a given pair of first and second bonding points has been connected by a wire, is set so that bonding is performed in one direction, either clockwise or counterclockwise. Generally, the leads 4 (41, 42 . . . 4N), the second bonding points, have a larger lead spacing in between in both structural terms and terms of disposition; as a result, the respective wires 5 that are connected to the leads are not parallel, and they as a whole are instead arranged in a fan-form configuration as shown in FIG. 4. Accordingly, the intermediate portion of the wire loop that is bonded to a pad 32 which is near the comer becomes closer to the pad 31 which is even nearer to the comer. Consequently, when bonding is performed by a wire to the comer pad 31 after a wire has been bonded to the pad 32 (which is next to the comer pad), the capillary 6 is likely to contact and deform the wire loop bonded to the pad 32 as indicated by a circle 6a (which represents the diameter of the axially intermediate portion of the capillary 6) as shown in FIG. 8.